The present invention generally relates to methods of improving surface morphology, and more particularly to a method of improving the surface morphology of a laser irradiated surface when carrying out a process such as planarization of a layer or changing a crystal state of a layer by irradiating a pulse laser on an interconnection material or the like of a semiconductor device.
For example, it is known from Semiconductor World, November 1988, p.84 to fill a via hole and to make a planarization of an interconnection using a pulse laser. According to Semiconductor World, a melted area is approximately 1.times.0.2 cm by one shot of the pulse laser having a pulse energy density of 1 to 10 J/cm.sup.2. When filling the via hole, for example, it is necessary to carry out a positioning so as to irradiate only the region which requires the pulse laser irradiation. For this reason, the pulse laser is generally irradiated over the entire chip to avoid the troublesome positioning.
According to the conventional pulse laser irradiation, the metal of the irradiated surface melts at the center portion of the one shot beam when the pulse energy density is in the range of 5 to 10 J/cm.sup.2 and higher. However, an energy density exists even within a one shot beam of the pulse laser, and the energy density at the peripheral portion of the one shot beam inevitably decreases. For this reason, the surface morphology of the irradiated surface deteriorates at the peripheral portion of the one shot beam.
FIG. 1 shows a region 1a which is irradiated by the one shot beam and a peripheral region 1b. When the irradiation by the one shot beam is successively carried out in the direction of the arrow, the planarization of the interconnection occurs within the region 1a but the surface morphology deteriorates in the peripheral region 1b which is indicated by a hatching. In the figures which follow, the region where the surface morphology is deteriorated will be indicated by the hatching, and the region where the planarization takes place will be indicated by white, that is, without the hatching.
When the via hole is to be filled and planarized, the beam size of the pulse laser becomes small in order to concentrate the energy of the pulse laser at the via hole. However, because it is difficult to accurately position the one shot beam at the via hole, it is inevitable to irradiate an area having a certain size. In addition, the irradiation area must be enlarged by overlapping the irradiating regions so as to eliminate non-irradiated regions. But when the irradiating regions are overlapped, the peripheral region of the previous or next one shot overlaps the peripheral region of the present one shot, thereby further deteriorating the surface morphology and enlarging the region in which the surface morphology is deteriorated.
When the above described deterioration is generated in the surface morphology of an interconnection layer, a disconnection of the interconnection may occur. Further, in the case of a multi-level interconnection, the surface morphology becomes poorer towards the upper layers of the multi-level interconnection, thereby increasing the chances of a disconnection of the interconnection. On the other hand, when the surface morphology is poor, there is also problem in that a high precision photolithography cannot be achieved.